Process for producing glass substrate for information recording medium and process for producing recording medium using said glass substrate

ABSTRACT

An object of the present invention is to prevent the adhesion of particles such as fine iron powders onto a glass substrate upon the production of a glass substrate for an information recording medium. To attain the above object, any one of the following means is adopted: (1) at least one of the production, inspection, packing and filling steps of the glass substrate for an information recording medium is carried out under a clean atmosphere which does not permit the adhesion of particles; (2) at least a wall surface of a chemical reinforcement tank containing a chemical reinforcement liquid therein and/or said retaining means which is contact with the chemical reinforcement liquid is formed of a stainless alloy having corrosion resistance in a high-temperature region as high as the heating temperature of the chemical reinforcement liquid; (3) a means for trapping fine particles existing in the chemical reinforcement liquid used in the chemical reinforcement step is provided; and (4) the glass substrate is washed with hydrochloric acid.

REFERENCE TO RELATED APPLICATION

[0001] This application claims the priority right of Japanese PatentApplication Nos. HEI 08-357543 filed on Dec. 20, 1996, HEI 08-357544filed on Dec. 30, 1996, HEI 08-357545 filed on Dec. 30, 1996 and HEI09-41513 filed on Feb. 9, 1997, the entire disclosure of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a process for producing aninformation recording medium used as a recording medium of aninformation processing apparatus and also a process for producing asubstrate employed for the medium.

[0004] 2. Description of the Related Art

[0005] One of such information recording mediums is a magnetic disk. Themagnetic disk is produced by forming a thin film such as a magneticlayer on a substrate and as the substrate for it, an aluminum or glasssubstrate has been employed. Reflecting the recent pursuit of highrecording densification, a glass substrate which makes it possible todecrease a distance between a magnetic head and a magnetic recordingmedium has come to be more prevalent compared with an aluminumsubstrate.

[0006] The glass substrate showing such an increasing tendency is, ingeneral, subjected to chemical reinforcement for heightening itsstrength so that it can endure the impact upon mounting on a magneticdisk driver. The surface of the glass substrate is polished with highaccuracy so that the flying height of the magnetic head can be loweredto the utmost, whereby the high recording densification is actualized.

[0007] As well as the improvement in the glass substrate, a magnetichead has been changed from a thin-film head to a magnetoresistive typehead (MR head) to meet the tendency to high recording densification.

[0008] As described above, high evenness on the surface of a magneticdisk is indispensable for the flying height reduction to heighten therecording density. In addition, when an MR head is employed, highflatness on the surface of a magnetic recording medium is also requiredin order to prevent the generation of TA (thermal asperity). The term“thermal asperity” as used herein means a phenomenon that a projectionexisting on the surface of a magnetic disk affects an MR head togenerate heat, thereby causing fluctuations in the resistance of thehead and causing a malfunction in the electromagnetic conversion.

[0009] There is an increasing demand for a magnetic disk having highevenness for both the lowering of flying height and prevention ofoccurrence of thermal asperity. A substrate having a highly even surfaceis eventually required for a magnetic disk having a surface of highevenness, but the requirement for heightening of the recording densityexceeds the level which can be materialized by only polishing thesurface of the substrate with high accuracy. In other words, it isimpossible to attain high evenness even by polishing with high accuracyif foreign matters adhere onto the substrate. The removal of foreignmatters has been carried out conventionally but the conventionaltolerance of foreign matters on the substrate is now regarded asinadequate for the recent tendency to heighten the recent densificationlevel.

[0010] Examples of such foreign matters include considerably fine ironpowders, stainless pieces, glass chips and organic substance whichcannot be removed by ordinary washing. When a thin film such as magneticfilm is stacked on a glass substrate without removing the particles suchas iron powders from the glass substrate, a protrusion is formed on thesurface of a magnetic disk, which becomes a cause for disturbing thelowering in the flying height or prevention of thermal asperity.

SUMMARY OF THE INVENTION

[0011] An object of the present invention is to prevent the adhesion ofparticles such as fine iron powders to a glass substrate. Another objectof the present invention is to produce an information recording mediumin a high yield by using a glass substrate from which foreign matters tobe a cause for under-layer defects have been removed.

[0012] The present inventors have conducted an extensive investigationon the cause of the adhesion of the particles such as fine iron powdersto a glass substrate. As a result, it has been found that a metal suchas iron powders or metal oxide generated from manufacturing facilitiesor buildings is floating in an atmosphere surrounding a chemicalreinforcement apparatus and such a floating substance is mixed in achemical reinforcement solution and adheres to the glass substrate.

[0013] In one aspect of the present invention, there is thus provided aprocess for producing a glass substrate for an information recordingmedium, which comprises a step of polishing a principal surface of aglass substrate and a step of chemically reinforcing said glasssubstrate (said step may hereinafter be called “chemical reinforcementstep”) by bringing it into contact with a chemical reinforcement liquidto substitute a portion of ions contained in said glass substrate byions having a diameter larger than the former ions in said chemicalreinforcement liquid, wherein at least one of the steps subsequent to afinal polishing step, that is, washing step, said chemical reinforcementstep, step of washing off said chemical reinforcement liquid, dryingstep, inspection step, and a step of packing said glass substrate orfilling the same in a container is effected in an atmosphere where cleanfiltered air is circulated so as to prevent the adhesion of particles tosaid glass substrate.

[0014] In a second aspect of the present invention, there is alsoprovided a process for producing a glass substrate for an informationrecording medium, which comprises a step of polishing a principalsurface of a glass substrate; and a step of chemically reinforcing saidglass substrate by bringing it into contact with a chemicalreinforcement liquid to substitute a portion of ions contained in saidglass substrate by ions having a diameter larger than the former ions insaid chemical reinforcement liquid, thereby reinforcing said glasssubstrate, wherein at least one of the steps subsequent to a finalpolishing step, that is, a washing step, said chemical reinforcementstep, a step of washing off said chemical reinforcement liquid, a dryingstep, an inspection step, and a step of packing said glass substrate orfilling the same in a container is effected under an environment whoseair cleanliness permits the existence of particles having a size of 0.3to 100 μm in an amount not more than 1000 particles per cubic feet·meterof the air.

[0015] In a third aspect of the present invention, there is alsoprovided a process for producing a glass substrate for an informationrecording medium as described in the first aspect, wherein the cleanfiltered air has cleanliness permitting the existence of particleshaving a size of 0.3 to 100 μm in an amount not more than 1000 particlesper cubic feet·meter of the air.

[0016] In a fourth aspect of the present invention, there is alsoprovided a process for producing a glass substrate for an informationrecording medium as described in the first or second aspect, wherein theair has cleanliness permitting the existence of particles in an amountnot more than 100 particles per cubic feet·meter of the air.

[0017] In a fifth aspect of the present invention, there is alsoprovided a process for producing a glass substrate for an informationrecording medium as described in the first or second aspect, wherein theair has cleanliness permitting the existence of particles in an amountnot more than 50 cubic feet·meter of the air.

[0018] In a sixth aspect of the present invention, there is alsoprovided a process for producing a glass substrate for an informationrecording medium as described in the first or second aspect, wherein thewashing and chemical reinforcement of the glass substrate subsequent tothe final polishing step are effected under an environment whose aircleanliness permits the existence of particles in a prescribed amount inthe air.

[0019] Examples of the particles to be removed in the present inventioninclude metal pieces such as fine iron powders, metal oxide pieces,glass chips, oil mist, silicon dust, fibers and organic substance.Particles large enough to cause inferiority during the use of aninformation recording medium or in the production step are removed. Thesize of the particles to be removed preferably ranges from 0.3 micron to100 microns, but control to a size within a range of from 1 micron to100 microns also brings about effects. Even the control of the size to10 to 100 microns does not cause problems in the practical use.

[0020] The steps subsequent to the final polishing such as washing,chemical reinforcement treatment, washing-off of the chemicalreinforcement solution, drying, inspection, or packing of the glasssubstrate may be carried out after covering, with a dust-tight cloth, apart of the manufacturing facilities or buildings which possiblygenerate dust; or may be carried out in a room having cleanlinesscontrolled to a predetermined degree by a clean booth method or cleanroom method, in order to reduce the particles, thereby heighteningcleanliness.

[0021] The step of reducing particles to heighten the cleanliness may becarried out at least one of the steps subsequent to the final polishingsuch as washing, chemical reinforcement treatment, washing-off of thechemical reinforcement solution, drying, inspection and packing of theglass substrate, but it is desired to carry out this step forheightening the cleanliness in all of these steps. It is particularlyeffective to reduce the particles, thereby improving cleanliness duringthe final polishing step to the chemical reinforcement step, becausewhen particles have adhered to the glass substrate during the chemicalreinforcement step, a portion of the glass substrate on which theparticles exist is not chemically reinforced or particles on the glasssubstrate firmly adhere to the glass substrate due to the chemicallyreinforced salt and cannot be removed easily by the subsequent washingstep. Accordingly, it is markedly important to maintain the cleanlinessof the glass substrate during the chemical reinforcement step or thestep therebefore.

[0022] The present inventors have carried out a further extensiveinvestigation on the cause of the adhesion of the fine iron powders to aglass substrate. As a result, it has been found that when the glasssubstrate is immersed in a chemical reinforcement tank containing achemical reinforcement solution therein or a chemical reinforcementsolution, a metal such as iron powders or a metal oxide adheres to theglass substrate directly or through the chemical reinforcement solutionfrom a retaining means for retaining the glass substrate. It has alsobeen found that since the chemical reinforcement treatment is effectedat a high temperature (ex. 350 to 450° C.), a chemically reinforcedglass substrate having a clean surface cannot be obtained unless aspecific stainless alloy is used.

[0023] In a seventh aspect of the present invention, there is thereforeprovided a process for producing a glass substrate for an informationrecording medium, which comprises a step of chemically reinforcing aglass substrate, which has been retained by a retaining means, bybringing its into contact with a chemical reinforcement liquid tosubstitute a portion of ions contained in said glass substrate by ionshaving a diameter larger than the former ions in said chemicalreinforcement liquid, wherein at least a wall surface of a chemicalreinforcement tank containing said chemical reinforcement liquid thereinand/or said retaining means for retaining said glass substrate which isin contact with said chemical reinforcement liquid is formed of astainless alloy having corrosion resistance in a high-temperature regionas high as the heating temperature of said chemical reinforcementliquid.

[0024] In an eighth aspect of the present invention, there is alsoprovided a process for producing a glass substrate for an informationrecording medium as described in the seventh aspect, wherein saidstainless alloy is a martensitic or austenitic stainless alloy.

[0025] Various shapes of retaining means can be considered as those forthe glass substrate. In short, preferred is a retaining means shaped tomake it possible to bring the glass substrate into contact with thechemical reinforcement solution under a predetermined condition and notto cause liquid sagging.

[0026] It is preferred to form both the chemical reinforcement tank andretaining means for the glass substrate from a martensitic or austeniticstainless alloy, but it is also possible to form either of them fromsuch an alloy. Moreover, it is preferred to form the whole portion ofthe chemical reinforcement tank or of the retaining means for the glasssubstrate from a martensitic or austenitic stainless alloy, but it ispossible to form only a portion which is brought into contact with thechemical reinforcement solution from such an alloy.

[0027] The martensitic or austenitic stainless alloy has excellentcorrosion resistance in a high temperature region so that it preventsthe generation of dust from metallic pieces. The chemical reinforcementsolution is presumed to be heated at 200 to 500° C.

[0028] The present inventors have carried out a still furtherinvestigation on the cause of the adhesion of fine iron powders, glasschips or stainless pieces to a glass substrate. As a result, it has beenfound that metal pieces such as iron powders generated from variousmanufacturing facilities or buildings installed around the chemicalreinforcement apparatus are mixed in the chemical reinforcement liquidand they are adhered to a glass substrate during the chemicalreinforcement of the glass substrate.

[0029] In a ninth aspect of the present invention, there is thereforeprovided a process for producing a glass substrate for an informationrecording medium, which comprises a step of chemically reinforcing aglass substrate by bringing it into contact with a chemicalreinforcement liquid to substitute a portion of ions contained in saidglass substrate by ions having a diameter larger than the former ions insaid chemical reinforcement liquid, wherein a means for trapping fineparticles existing in said chemical reinforcement liquid is provided.

[0030] In a tenth aspect of the present invention, there is alsoprovided a process for producing a glass substrate for an informationrecording medium as described in the ninth aspect, wherein said meansfor trapping fine particles is a filter for filtering said chemicalreinforcement liquid circulated.

[0031] In an eleventh aspect of the present invention, there is alsoprovided a process for producing a glass substrate for an informationrecording medium as described in the ninth aspect, wherein said fineparticles are fine iron powders and said trapping means is a magnetdisposed to be in contact with said chemical reinforcement liquid.

[0032] As a chemical reinforcement method of the present invention,preferred is low-temperature type chemical reinforcement in which an ionexchange is performed in a region not exceeding the glass transitionpoint. Examples of an alkali fusion salt used as a chemicalreinforcement solution include potassium nitrate, sodium nitrate, and amixture thereof.

[0033] Examples of the glass substrate usable in the present inventioninclude aluminosilicate glass, soda-lime glass and crystallized glass.

[0034] As particles, metal pieces such as iron powders or stainless,metal oxides, glass chips and organic substance can be given asexamples. The removal of such particles having a particle size ofseveral microns or greater is effective for the process of the presentinvention.

[0035] Examples of the trapping means of the fine particles in thepresent invention include a filter. Any filters that can supply thefiltered chemical reinforcement liquid can be employed. For example, amicro-sieve (a wire mesh having pores made by etching) can be used. Inthis case, the chemical reinforcement solution is heated at a hightemperature so that the use of a martensitic or austenitic stainlessalloy having excellent corrosion resistance is preferred.

[0036] As another means for trapping particles, a magnet or the like maybe disposed so as to be in contact with the chemical reinforcementsolution, in the case where the particles are fine iron powders.

[0037] The trapping means may be fixed or set movable.

[0038] The present inventors have carried out a further extensiveinvestigation. As a result, it has been found that by washing the glasssubstrate with hydrochloric acid, foreign matters can be satisfactorilyremoved, particularly, iron contaminants dissolved therein can beremoved effectively.

[0039] In the twentieth aspect of the present invention, there istherefore provided a process for producing a glass substrate for aninformation recording medium, said glass substrate having a principalsurface subjected to precision polishing, which comprises a step ofwashing said glass substrate with hydrochloric acid.

[0040] In the thirteenth aspect of the present invention, there is alsoprovided a process for producing a glass substrate for an informationrecording medium as described in the twentieth aspect, which comprises astep of chemically reinforcing a glass substrate by substituting aportion of ions contained in said glass substrate by ions having adiameter larger than the former ions, wherein said glass substrate iswashed with hydrochloric acid as a pre-treatment or post-treatment stepof the chemical reinforcement step.

[0041] In the present invention, by washing with hydrochloric acid,particularly iron contaminants dissolved therein can be removedeffectively and also other foreign matters, for example, nickel,stainless, chromium, oxides or organic substance thereof can be removed.Washing with hydrochloric acid may be carried out between any two stepsfor the production of a glass substrate for an information recordingmedium but it is effectively carried out in one step or plural stepsbetween the polishing step-and packing step of the glass substratecompleted. In the process for producing a glass substrate for aninformation recording medium including a chemical reinforcement step,when the step of washing with hydrochloric acid is carried out prior tothe chemical reinforcement step, remaining of an unreinforced portioncaused by foreign matters on the surface of the glass substrate can beprevented effectively. Alternatively, it is also possible to carry outthe chemical reinforcement step and the step of washing off the fusionsalt or the like in a clean booth or by the like method under anatmosphere where the air of a high cleanliness has been circulated andto carry out the step of washing with hydrochloric acid prior to packingof the glass substrate.

[0042] It is preferred to wash the whole portion of the glass substratewith hydrochloric acid but, the principal surface and the outer andinner peripheral sides of the glass substrate may be selectively washedin order to prevent under-layer defects and in order to prevent thetransfer of the iron contaminants or the like, which has been adhered onthe end face, to the principal surface, respectively. Examples of thewashing method include dipping of a glass substrate in hydrochloric acidand spraying hydrochloric acid onto a glass substrate. Effects ofwashing with hydrochloric acid can be enhanced by applying supersonicwaves during the washing with hydrochloric acid; or by scrub washingduring or after washing with hydrochloric acid. As hydrochloric acid,1-12N (N:Normality) dilute hydrochloric acid or concentratedhydrochloric acid is preferred. The temperature of hydrochloric acid isadjustable in the range of 30° C. to 60° C. as appropriate. Theprocessing time of hydrochloric washing is adjustable in the range of 1minute to 20 minutes as appropriate.

[0043] Specific examples of the glass substrate for an informationrecording medium include a glass substrate for a magnetic disk.

[0044] In the fourteenth aspect of the present invention, there istherefore provided a process for producing a glass substrate for aninformation recording medium as described in any one of the first,second, seventh, ninth and twelfth aspects, wherein said glass substratefor an information recording medium is a glass substrate for a magneticdisk.

[0045] In the fifteenth aspect of the present invention, there is alsoprovided a process for producing a glass substrate for an informationrecording medium as described in the fourteenth aspect, wherein saidglass substrate for a magnetic disk is a glass substrate for a magneticdisk for a magnetoresistive type head.

[0046] In the sixteenth aspect of the present invention, there is alsoprovided a process for producing an information recording medium, whichcomprises forming at least a recording layer on a glass substrateobtained by the process for producing a glass substrate for aninformation recording medium as described in any one of the first,second, seventh, ninth and twentieth aspects.

[0047] In a seventeenth aspect of the present invention, there is alsoprovided a process for producing an information recording medium asdescribed in the sixteenth aspect, wherein said recording layer is amagnetic layer.

[0048] In the present invention, no particular limitation is imposed onthe kind, size, thickness or the like of a glass substrate. Examples ofthe material quality of the glass substrate include aluminosilicateglass, soda-lime glass, soda aluminosilicate glass, aluminoborosilicateglass, borosilicate glass, quartz glass, chain silicate glass and glassceramics such as crystallized glass.

[0049] As an aluminosilicate glass, preferred is a glass for chemicalreinforcement which contains as principal components 62 to 75 wt. % ofSiO₂, 5 to 15 wt. % of Al₂O₃, 4 to 10 wt. % of Li₂O, 4 to 12 wt. % ofNa₂O and 5.5 to 15 wt. % of ZrO₂; and wherein the weight ratio of Na₂Oto ZrO₂ is 0.5 to 2.0 and that of Al₂O₃ to ZrO₂ is 0.4 to 2.5.

[0050] It is preferred to use a glass for chemical reinforcementcontaining 57 to 74% of SiO₂, 0 to 2.8% of ZrO₂, 3 to 15% of Al₂O₃, 7 to16% of Li₂O and 4 to 14% of Na₂O, each in terms of mole %, in order toavoid the appearance of protrusions on the surface of the glasssubstrate which results from the undissolved substance of ZrO₂.

[0051] The aluminosilicate glass or the like having such a compositionhas increased bending strength, a deepened compression stress layer andimproved Knoop hardness.

[0052] In the present invention, for the improvement of impactresistance and vibration resistance, chemical reinforcement treatment isapplied to the surface of a glass substrate by the low-temperature ionexchange method.

[0053] No particular limitation is imposed on the chemical reinforcementmethod insofar as it is the chemical reinforcement method well known inthe art. Preferred is low-temperature type chemical reinforcement inwhich ion exchange is effected in a temperature range not exceeding theglass transition point. Examples of the alkali fusion salt used forchemical reinforcement include potassium nitrate and sodium nitrate, anda mixture thereof.

[0054] The glass substrate for an information recording medium accordingto the process of the present invention can be used as a glass substratefor a magnetic recording medium, a glass substrate for an opticalmagnetic disk and a glass substrate for an electro-optical disk.

[0055] In particular, the glass substrate can be used suitably as amagnetic disk substrate for a magnetoresistive type head (includinglarge-sized magnetoresistive type head) which carries out recording andreproduction and it can also be used suitably for a process forproducing an information recording medium by using the substrate.

[0056] The magnetic recording medium of the present invention willhereinafter be described.

[0057] The magnetic recording medium of the present invention has atleast a magnetic layer formed on the glass substrate for a magneticrecording medium according to the present invention.

[0058] Particles causing thermal asperity or head crash are notgenerated in the present invention so that protrusions, which willotherwise be formed by the particles, do not appear on the principalsurface of the glass substrate at the time when a magnetic recordingmedium is produced by forming a magnetic layer on the glass substrate,whereby the prevention of a head crash on a still higher level can beattained. Particularly for a magnetic recording medium which carries outreproduction by a magnetoresistive type head, absence of protrusionsmakes it possible to fully exhibit the function of the magnetoresistivetype head. Also for a CoPt type magnetic recording medium which issuitably used for a magnetoresistive type head, absence of protrusionsmakes it possible to fully exhibit the performance of the head.

[0059] Also on the recording or reproducing surface of the magneticrecording medium, no protrusion causing thermal asperity is formed,whereby the prevention of head crash on a still higher level can beeffected.

[0060] Furthermore, since no particles which will cause thermal asperityare generated, neither defects in a film such a magnetic layer norerrors occur.

[0061] The magnetic recording medium is fabricated by stacking a primarylayer, magnetic layer, protective layer and lubricating layersuccessively on a glass substrate for a magnetic disk which has apredetermined evenness and surface roughness and has a surface subjectedto chemical reinforcement as needed.

[0062] The primary layer of the magnetic recording medium according tothe present invention may be selected, depending on the magnetic layerstacked thereon.

[0063] Examples of the primary layer include those composed of at leastone material selected from nonmagnetic metals such as Cr, Mo, Ta, Ti, W,V, B and Al. When the magnetic layer is composed mainly of Co, theprimary layer is preferably formed of a single substance or alloy of Crfrom the viewpoint of the improvement in the magnetic characteristics.The primary layer is not limited to a single layer but may be formed ofplural layers of the same material or different materials. Examples ofthe multi-layered primary layer include Cr/Cr, Cr/CrMo, Cr/CrV, CrV/CrV,Al/Cr/CrMo, Al/Cr/Cr, Al/Cr/CrV and Al/CrV/CrV.

[0064] No particular limitation is imposed on the material of themagnetic layer in the magnetic recording medium of the presentinvention.

[0065] Examples of the magnetic layer include magnetic layers composedmainly of Co such as CoPt, CoCr, CoNi, CoNiCr, CoCrTa, CoPtCr, CoNiPt,CoNiCrPt, CoNiCrTa, CoCrTaPt and CoCrPtSiO. Alternatively, the magneticlayer may have a multi-layered structure (ex. CoPtCr/CrMo/CoPtCr,CoCrTaPt/CrMo/CoCrTaPt) which aims at a noise reduction by dividing themagnetic film by a nonmagnetic film (ex. Cr, CrMo, CrV).

[0066] Examples of the magnetic layer for a magnetoresistive type head(MR head) or giant magnetoresistive type head (GMR head) include Coalloys in which an impurity element selected from Y, Si, rare earthelements, Hf, Ge, Sn and Zn or an oxide of such an impurity element hasbeen incorporated.

[0067] The magnetic layer may have a granular structure in whichmagnetic particles such as Fe, Co, FeCo or CoNiPt, have been dispersedin a non-magnetic layer formed of ferrite, iron—rare earth element, SiO₂or BN. In addition, the recording system of the magnetic layer may beeither an internal recording or a vertical recording system.

[0068] No particular limitation is imposed on the protective layer inthe magnetic recording medium of the present invention.

[0069] Examples of the protective layer include Cr film, Cr alloy film,carbon film, zirconium film and silica film. The protective layer can beformed continuously with a primary layer and magnetic layer by anin-line sputtering apparatus. The protective layer may be a single layeror a multi-layer formed of the same material or different materials.

[0070] On the above-described protective layer or instead of theabove-described protective layer, another protective layer may be formedin the present invention. For example, it is possible to dispersecolloidal silica particles in a diluted solution of tetraalkoxysilane inan alcohol solvent and apply the resulting dispersion to a Cr film,followed by calcination, whereby a silicon oxide (SiO₂) film is formedinstead of the above-described protective layer.

[0071] No particular limitation is imposed on the lubricating layer inthe magnetic recording medium of the present invention.

[0072] The lubricating layer is formed, for example, by dilutingperfluoropolyether (PFPE), which is a liquid lubricant, with a fluorenebase solvent and applying the resulting diluted solution onto thesurface of the medium by the dipping method, spin coating method orspraying method and optionally heating the coated surface.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0073] The present invention will hereinafter be described morespecifically based on the examples.

EXAMPLE 1

[0074] (1) Rough Grinding Step

[0075] A glass substrate formed of an aluminosilicate glass cut from asheet glass, which had been formed by the down-drawing method, into adisk having a diameter of 96 mmØ and a thickness of 3 mm by a grindingstone was ground by a comparatively rough diamond grinder into a diskhaving a diameter of 96 mmØ and a thickness of 1.5 mm.

[0076] Instead of the down-drawing method, it is possible to employ thedirect pressing method to press a fused glass into a glass disk by usingan upper mold, lower mold and cylindrical mold. Alternatively, thefloating method may be used.

[0077] Incidentally, as the aluminosilicate glass, used was a glass forchemical reinforcement containing as principal components 57 to 74% ofSiO₂, 0 to 2.8% of ZnO₂, 3 to 15% of Al₂O₃, 7 to 16% of Li₂O and 4 to 14wt. % of Na₂O, each in terms of mole % (for example, a glass forchemical reinforcement containing as principal components 67.0% of SiO₂,1.0% of ZrO₂, 9.0% of Al₂O₃, 12.0% of Li₂O and 10.0% of Na₂O, each interms of mole %).

[0078] Both sides of the glass substrate were ground by a diamondgrinder having a particle size smaller than the above grinder,respectively. The load at that time was about 100 kg, by which bothsides of the glass substrate were each formed to have a surfaceroughness of about 10 μm in terms of Rmax (as measured in accordancewith JIS B 0601).

[0079] By using a cylindrical grinding stone, a hole was made at thecenter of the glass substrate and the outer peripheral end face wasground so that the glass substrate had a diameter of 95 mmØ. The outerperipheral end face and inner peripheral face were subjected to apredetermined chamfering treatment. The end face of the glass substrateat that time had a surface roughness of about 4 μm in terms of Rmax.

[0080] (2) Mirror-Finish Processing Step of End Face

[0081] While the glass surface was allowed to rotate, its end face wassubjected to brush polishing to have a surface roughness of about 1 μmin terms of Rmax and about 0.3 μm in terms of Ra.

[0082] After the completion of the mirror-finish processing of the endface, the surface of the glass substrate was washed with water.

[0083] (3) Lapping Step

[0084] The glass substrate was then subjected to a lapping step. Thelapping step aims at the improvement in the size accuracy and shapeaccuracy. The lapping was carried out twice by a lapping apparatus bychanging the grain size of an abrasive from #400 to #1000.

[0085] Described specifically, with an alumina abrasive having aparticle size of #400 at a load L of about 100 kg, both sides of theglass substrate placed in a carrier were lapped to have an arealaccuracy of 0 to 1 μm and a surface roughness (Rmax) of 6 μm or so byallowing the inner gear and outer gear to rotate.

[0086] The lapping was then effected by changing the grain size of thealumina abrasive to #1000, whereby the surface roughness was improved toabout 2 μm (Rmax).

[0087] After the completion of the lapping, the glass substrate wasimmersed and washed successively in the washing tanks of a neutraldetergent and water.

[0088] (4) First Polishing Step

[0089] The glass substrate was then subjected to a first polishing step.The first polishing step was carried out by a polishing machine in orderto remove the mars or distortions which had remained after the lappingstep.

[0090] Described specifically, the first polishing step was carried outunder the following polishing conditions by using a rigid polisher(“Cerium pad MHC15”, trade name; product of Speed Pham) as a polisher(polishing powder).

[0091] Polishing liquid: cerium oxide+water

[0092] Load: 300 g/cm² (L=238 kg)

[0093] Polishing time: 15 minutes

[0094] Removed amount: 30 μm

[0095] Rotational speed of lower platen: 40 rpm

[0096] Rotational speed of upper platen: 35 rpm

[0097] Rotational speed of inner gear: 14 rpm

[0098] Rotational speed of outer gear: 29 rpm

[0099] After completion of the first polishing step, the glass substratewas immersed successively in the washing tanks of a neutral detergent,pure water, pure water, IPA (isopropyl alcohol) and IPA (vapor drying),whereby washing was carried out.

[0100] (5) Second Polishing Step

[0101] A second polishing step was carried out using a polishing machineemployed in the first polishing step and a soft polisher (“Polirax”,trade name; product of Speed Pham) instead of the rigid polisher.Polishing was conducted under the similar conditions to the firstpolishing step except that the load, polishing time and removed amountwere changed to 100 g/cm², 5 minutes and 5 μm, respectively.

[0102] Steps subsequent to the second polishing step were carried outunder an atmosphere where the air filtered through a clean booth wasbeing circulated. Described specifically, all the steps after the secondpolishing step, that is, steps of washing, chemical reinforcement,washing-off of the chemical reinforcement solution, vapor drying andpacking of the substrate were carried out under the environment whoseair cleanliness permits the existence of particles of 2 to 4 μm in anamount of 800 particles per one cubic feet·meter.

[0103] After the completion of the second polishing step, the glasssubstrate was immersed and washed successively in the washing tanks of aneutral detergent, a neutral detergent, pure water, pure water, IPA(isopropyl alcohol) and IPA (vapor drying), respectively. Incidentally,supersonic waves were applied to each of the washing tanks.

[0104] (6) Chemical Reinforcement Step

[0105] After the completion of the grinding, polishing and washingsteps, the glass substrate was subjected to chemical reinforcement.

[0106] The glass substrate which had been washed was heated in advanceto 300° C. and immersed for about 3 hours in a chemical reinforcementsolution preliminarily heated to 400° C., said solution having beenprepared by mixing potassium nitrate (60%) and sodium nitrate (40%).With a view to applying the chemical reinforcement all over the surfaceof the glass substrate, the chemical reinforcement was effected withplural glass substrates being placed in a holder upon immersion so as toretain them at the end faces.

[0107] When the glass substrate is immersed in the chemicalreinforcement solution, lithium ions and sodium ions on the surfacelayer of the glass substrate are substituted by sodium ions andpotassium ions in the chemical reinforcement solution, respectively,whereby the glass substrate is reinforced.

[0108] The compression stress layer formed on the surface layer of theglass substrate had a thickness of about 100 to 200 μm.

[0109] After the completion of the chemical reinforcement, the glasssubstrate was quenched by immersing it in a water tank of 20° C. andkept it in the tank for about 10 minutes. The glass substrate was thensubjected to vapor drying and surface inspection, followed by filling ina case, whereby the production steps were completed.

[0110] After the completion of the quenching, the glass substrate wasimmersed in sulfuric acid heated to about 40° C. Washing was effected,while supersonic waves were applied.

[0111] The principal surface of the glass substrate so obtained had asurface roughness Ra of 0.5 to 1 nm.

[0112] As a result of close examination of the glass surface, noparticles which would be a cause for thermal asperity were observed.Described specifically, a glass substrate having a diameter of 65 mm wasleft alone for 30 minutes and then inspected by a laser-scatter typesurface defect inspection instrument. As a result, the number of theparticles was found to be 30 or less per micron. It is preferred thatthe particles of at least 5 μm have been removed from the surface of theglass substrate.

[0113] Incidentally, a glass substrate produced according to theconventional process contained several hundreds of particles, mainlyiron powders, having a particle size of 10 to 100 micron.

[0114] (7) Fabrication Step of a Magnetic Disk

[0115] On both sides of the glass substrate for a magnetic disk obtainedthrough the above steps, an AlN texture layer, Cr primary layer, CrMoprimary layer, CoPtCrTa magnetic layer and C protective layer wereformed successively by an in-line system sputtering apparatus, whereby amagnetic disk was obtained.

[0116] As a result of the grinding test on the magnetic disk soobtained, neither hit (light touch of the head with the surface of themagnetic disk) nor crash (collision of the head with the protrusion onthe surface of the magnetic disk) was observed. It was also confirmedthat no defects were caused in the film such as magnetic layer by theparticles which would otherwise be a cause for thermal asperity. In thepresent example, a magnetic disk was thus fabricated in a high yield.

[0117] After the grinding test, the magnetic disk was subjected to areproduction test by a magnetoresistive type head. As a result, amalfunction in the reproduction attributable to thermal asperity was notobserved from each of the plural samples (500 disks).

EXAMPLE 2

[0118] On both sides of the glass substrate for a magnetic disk, whichhad been obtained in Example 1, were formed by an in-line sputteringapparatus a primary layer composed of Al (film thickness: 50 angstrom),Cr (1000 angstrom) and CrMo (100 angstrom); a magnetic layer composed ofCoPtCr (120 angstrom), CrMo (50 angstrom) and CoPtCr (120 angstrom); anda protective layer composed of Cr (50 angstrom).

[0119] The resulting substrate was immersed in a solution of anorganosilicon compound (a mixed solution of water, IPA andtetraethoxysilane) having fine silica particles (particle size: 100angstrom) dispersed therein, followed by calcination, whereby aprotective layer composed of SiO₂ and having a texture function wasformed. The surface of the protective layer was then subjected to diptreatment with a lubricant composed of perfluoropolyether to form alubricating layer thereon, whereby a magnetic disk for an MR head wasobtained.

[0120] As a result of the grinding test on the magnetic disk sofabricated, neither hit nor crash was observed. It was also confirmedthat no defect occurred in the film such as magnetic layer. As a resultof the reproduction test by using a magnetoresistive type head, amalfunction in the reproduction attributable to thermal asperity was notobserved.

[0121] Moreover, thermal asperity, which would otherwise occur duringthe use after a non-defective magnetic disk was installed in a magneticdisk driver, did not occur.

EXAMPLE 3

[0122] In a similar manner to Example 1 or Example 2 except that theprimary layer was formed of Al, Cr and Cr and the magnetic layer wasformed of CoNiCrTa, a magnetic disk for a thin-film head was fabricated.

[0123] The magnetic disc so obtained was confirmed to show the sameresults as in the above examples.

EXAMPLE 4

[0124] (1) Rough Grinding Step

[0125] A glass substrate formed of an aluminosilicate glass cut from asheet glass, which had been formed by the down-drawing method, into adisk having a diameter of 96 mmØ and a thickness of 3 mm by a grindingstone was ground by a comparatively rough diamond grinder into a diskhaving a diameter of 96 mmØ and a thickness of 1.5 mm.

[0126] Instead of the down-drawing method, it is possible to employ thedirect pressing method to press the fused glass into a glass disk by anupper mold, lower mold and cylindrical mold. Alternatively, floatingmethod may be adopted for the formation of a glass disk.

[0127] Incidentally, as the aluminosilicate glass, used was a glass forchemical reinforcement containing as principal components 57 to 74% ofSiO₂, 0 to 2.8% of ZnO₂, 3 to 15% of Al₂O₃, 7 to 16% of Li₂O and 4 to 14wt. % of Na₂O, each in terms of mole %, (for example, a glass forchemical reinforcement containing as principal components 67.0% of SiO₂,1.0% of ZnO₂, 9.0% of Al₂O₃, 12.0% of Li₂O and 10.0% of Na₂O, each interms of mole %).

[0128] Both sides of the glass substrate were ground by a diamondgrinder having a particle size smaller than the above grinder. The loadat that time was about 100 kg, by which the both sides of the glasssubstrate were formed to have a surface roughness of about 10 μm interms of Rmax (as measured in accordance with JIS B 0601).

[0129] By using a cylindrical grinding stone, a hole was made at thecenter of the glass substrate and the outer peripheral end face wasground to have a diameter of 95 mmØ. The outer peripheral end face andinner peripheral face were subjected to a predetermined chamferingtreatment. The end face of the glass substrate at that time had asurface roughness of about 4 μm in terms of Rmax.

[0130] (2) Mirror-Finish Processing Step of End Face

[0131] While the glass surface was allowed to rotate, the end face ofthe glass substrate was subjected to brush polishing to a surfaceroughness of about 1 μm in terms of Rmax and about 0.3 μm in terms ofRa.

[0132] After the completion of the mirror-finish processing of the endface, the surface of the glass substrate was washed with water.

[0133] (3) Lapping Step

[0134] The glass substrate was then subjected to a lapping treatment.The lapping step aims at the improvement in the size accuracy and shapeaccuracy. The lapping was carried out by a lapping apparatus twice bychanging the grain size of an abrasive from #400 and #1000.

[0135] Described specifically, with an alumina abrasive having aparticle size of #400 at a load L of about 100 kg, both sides of theglass substrate placed in a carrier were lapped to have an arealaccuracy of 0 to 1 μm and a surface roughness (Rmax) of 6 μm or so byallowing the inner gear and outer gear to rotate.

[0136] The lapping was then effected by changing the grain size of thealumina abrasive to #1000, whereby the surface roughness was improved toabout 2 μm (Rmax).

[0137] After the completion of the lapping, the glass substrate wasimmersed and washed successively in the washing tanks of a neutraldetergent and water.

[0138] (4) First Polishing Step

[0139] The glass substrate was then subjected to a first polishing step.The first polishing step was carried out by a polishing machine in orderto remove the mars or distortions which had remained after the lappingstep.

[0140] Described specifically, the first polishing step was carried outunder the following polishing conditions by using a rigid polisher(“Cerium pad MHC15”, trade name; product of Speed Pham) as a polisher(polishing powder).

[0141] Polishing liquid: cerium oxide+water

[0142] Load: 300 g/cm² (L=238 kg)

[0143] Polishing time: 15 minutes

[0144] Removed amount: 30 μm

[0145] Rotational speed of lower platen: 40 rpm

[0146] Rotational speed of upper platen: 35 rpm

[0147] Rotational speed of inner gear: 14 rpm

[0148] Rotational speed of outer gear: 29 rpm

[0149] After completion of the first polishing step, the glass substratewas immersed successively in the washing tanks of a neutral detergent,pure water, pure water, IPA (isopropyl alcohol) and IPA (vapor drying),whereby washing was carried out.

[0150] (5) Second Polishing Step

[0151] A second polishing step was carried out using a polishing machineemployed in the first polishing step and a soft polisher (“Polirax”,trade name; product of Speed Pham) instead of the rigid polisher.Polishing was conducted under the similar conditions to the firstpolishing step except that the load, polishing time and removed amountwere changed to 100 g/cm², 5 minutes and 5 μm, respectively.

[0152] After the completion of the second polishing step, the glasssubstrate was immersed and washed successively in the washing tanks of aneutral detergent, a neutral detergent, pure water, pure water, IPA(isopropyl alcohol) and IPA (vapor drying), respectively. Incidentally,supersonic waves were applied to each of the washing tanks.

[0153] (6) Chemical Reinforcement Step

[0154] After the completion of the grinding, polishing and washingsteps, the glass substrate was subjected to chemical reinforcement. Thechemical reinforcement is carried out by pouring a chemicalreinforcement solution into a chemical reinforcement tank and thenimmersing the glass substrate supported by a retaining member in thechemical reinforcement solution. Incidentally, the retaining member forthe glass substrate is formed of three supports which form a pluralnumber of V-grooves at equal intervals in the arranging direction of theglass substrate, said three supports being connected at each end facethereof with a connecting member. Each glass substrate is supported andretained at three points by the V-grooves in the same plane of threesupports and plural glass substrates are arranged in the extendingdirection of these supports.

[0155] The supports and connecting members of the retaining member ofthis example are each formed of SUS316 which is an austenitic stainlessalloy having excellent corrosion resistance in a high temperatureregion. The chemical reinforcement tank is, on the other hand, formed ofSUS304 which is also an austenitic stainless alloy. The chemicalreinforcement tank and retaining means may be formed of the samematerial or different materials. As another stainless alloy, SUS316L issuited, for example. The above stainless alloys (SUS316, SUS304,SUS316L) are defined by Japanese Industrial Standards (JIS G 4303-JIS G4305).

[0156] Described specifically, the chemical reinforcement was carriedout as follows. The glass substrate which had been washed was heated inadvance to 300° C. and immersed in a chemical reinforcement solutionpreliminarily heated to 400° C. for about 3 hours, said solution havingbeen prepared by mixing potassium nitrate (60%) and sodium nitrate(40%). With a view to applying the chemical reinforcement all over thesurface of the glass substrate, the chemical reinforcement was effectedwith plural glass substrates being placed in a holder upon immersion soas to retain them at the end faces.

[0157] When the glass substrate is immersed in the chemicalreinforcement solution, lithium ions and sodium ions on the surfacelayer of the glass substrate are substituted by sodium ions andpotassium ions in the chemical reinforcement solution, respectively,whereby the glass substrate is reinforced.

[0158] The compression stress layer formed on the surface layer of theglass substrate had a thickness of about 100 to 200 μm. Upon chemicalreinforcement, each of the chemical reinforcement tank and the retainingmember for the glass substrate which was in contact with the hotchemical reinforcement solution was formed of an austenitic stainlessalloy having excellent chemical durability so that adhesion of metalpieces such as iron powders or chromium and metal oxides to the glasssubstrate could be prevented.

[0159] After the completion of the chemical reinforcement, the glasssubstrate was quenched by dipping in a water tank of 20° C. and kept init for about 10 minutes.

[0160] After the completion of the quenching, the glass substrate wasimmersed in sulfuric acid heated to about 40° C. It was washed, whilesupersonic waves were applied.

[0161] The principal surface of the glass substrate so obtained had asurface roughness Ra of 0.5 to 1 nm.

[0162] As a result of close examination of the glass surface, particleswhich would be a cause for thermal asperity were not observed. Inparticular, fine iron powders having a particle size of at least 3 to 5μm were not observed at all.

[0163] (7) Fabrication Step of Magnetic Disk

[0164] On both sides of the glass substrate for a magnetic disk obtainedthrough the above steps, an AlN texture layer, Cr primary layer, CrMoprimary layer, CoPtCrTa magnetic layer and C protective layer wereformed successively by an in-line system sputtering apparatus, whereby amagnetic disk was obtained.

[0165] As a result of the grinding test on the magnetic disk soobtained, neither hit nor crash was observed. It was also confirmed thatno defects were caused in the film such as magnetic layer by theparticles which would otherwise cause thermal asperity.

[0166] Incidentally, the present example wherein the tank and theretaining member in contact with the chemical reinforcement solutionwere each formed of a stainless alloy having high corrosion resistancewas compared with a comparative example wherein the retaining member andtank were formed of a stainless alloy such as SUS430 (ferrite basestainless alloy) having not so high corrosion resistance. As a result,in the comparative example, many fine iron powders having a particlesize of 10 to 80 microns were observed on the surface of the glasssubstrate. Thus, the amount of foreign matters such as iron was markedlysmall in the present example.

[0167] After the grinding test, the magnetic disk of the present examplewas subjected to a reproduction test by using a magnetoresistive typehead. As a result, a malfunction in reproduction attributable to thermalasperity was not observed from each of the plural samples (500 disks).

EXAMPLES 5 to 6

[0168] In a similar manner to Example 4 except for the use of asoda-lime glass (Example 5) or a soda aluminosilicate glass (Example 6)instead of the aluminosilicate glass, a glass substrate for a magneticdisk and a magnetic disk were fabricated.

[0169] As a result, a chemically reinforced glass having a surface freefrom metallic pieces such as iron powders was obtained as in Example 4.

[0170] In a similar manner to Example 4 except that as the material forthe chemical reinforcement tank and retaining member, a martensiticstainless alloy was used instead of the austenitic stainless alloy, theglass substrate was subjected to chemical reinforcement. Same resultswere obtained as in the case of the autstenitic stainless alloy.

EXAMPLE 7

[0171] On both sides of the glass substrate for a magnetic disk obtainedin Example 4, formed were a primary layer composed of Al (filmthickness: 50 angstrom), Cr (1000 angstrom) and CrMo (100 angstrom), amagnetic layer composed of CoPtCr (120 angstrom), CrMo (50 angstrom) andCoPtCr (120 angstrom) and a protective layer composed of Cr (50angstrom).

[0172] The substrate so obtained was immersed in a solution of anorganosilicon compound (a mixed solution of water, IP andtetraethoxysilane) having fine silica particles (particle size: 100angstrom) dispersed therein, followed by calcination, whereby aprotective layer composed of SiO₂ and having a texture function wasformed. The surface of the protective layer was then subjected to diptreatment with a lubricant composed of perfluoropolyether to form alubricating layer, whereby a magnetic disk for an MR head was obtained.

[0173] As a result of a grinding test on the magnetic disk sofabricated, neither hit nor crash was observed. It was also confirmedthat no defects occurred in the magnetic layer or the like. As a resultof the reproduction test by a magnetoresistive type head, a malfunctionin reproduction attributable to thermal asperity was not observed.

EXAMPLE 8

[0174] In a similar manner to Example 7 except that the primary layerwas formed of Al, Cr and Cr and the magnetic layer was formed ofCoNiCrTa, a magnetic disk for a thin-film head was obtained.

[0175] The magnetic disk so obtained was confirmed to show the sameresults as in Example 7.

EXAMPLE 9

[0176] (1) Rough Grinding Step

[0177] A glass substrate formed of an aluminosilicate glass cut from asheet glass, which had been formed by the down-drawing method, into adisk having a diameter of 96 mmØ and a thickness of 3 mm by a grindingstone was ground by a comparatively rough diamond grinder to a diskhaving a diameter of 96 mmØ and a thickness of 1.5 mm.

[0178] Instead of the down-drawing method, it is possible to employ thedirect pressing method to press a fused glass into a glass disk by usingan upper mold, lower mold and cyrindrical mold. Alternatively, thefloating method may be used.

[0179] Incidentally, as the aluminosilicate glass, used was a glass forchemical reinforcement containing as principal components 57 to 74% ofSiO₂, 0 to 2.8% of ZnO₂, 3 to 15% of Al₂O₃, 7 to 16% of Li₂O and 4 to 14wt. % of Na₂O, each in terms of mole %, (for example, a glass forchemical reinforcement containing as principal components 67.0% of SiO₂,1.0% of ZrO₂, 9.0% of Al₂O₃, 12.0% of Li₂O and 10.0% of Na₂O, each interms of mole %).

[0180] The both sides of the glass substrate were ground by a diamondgrinder having a particle size smaller than the above grinder,respectively. The load at that time was about 100 kg, by which the bothsides of the glass substrate were each formed to have a surfaceroughness of about 10 μm in terms of Rmax (as measured in accordancewith JIS B 0601).

[0181] By using a cylindrical grinding stone, a hole was made at thecenter of the glass substrate and the outer peripheral end face wasground so that the glass substrate had a diameter of 95 mmØ. The outerperipheral end face and inner peripheral face were subjected to apredetermined chamfering treatment. The end face of the glass substrateat that time had a surface roughness of about 4 μm in terms of Rmax.

[0182] (2) Mirror-Finish Processing Step of End Face

[0183] While the glass surface was allowed to rotate, its end face wassubjected to brush polishing to have a surface roughness of about 1 μmin terms of Rmax and about 0.3 μm in terms of Ra.

[0184] After the completion of the mirror-finish processing of the endface, the surface of the glass substrate was washed with water.

[0185] (3) Lapping Step

[0186] The glass substrate was then subjected to a lapping step. Thelapping step aims at the improvement in the size accuracy and shapeaccuracy. The lapping was carried out by a lapping apparatus twice bychanging the grain size of an abrasive from #400 and #1000.

[0187] Described specifically, with an alumina abrasive having aparticle size of #400 at a load L of about 100 kg, both sides of theglass substrate placed in a carrier were lapped to have an arealaccuracy of 0 to 1 μm and surface roughness (Rmax) of 6 μm or so byallowing the inner gear and outer gear to rotate.

[0188] The lapping was then effected by changing the grain size of analumina abrasive to #1000, whereby the surface roughness was improved toabout 2 μm (Rmax).

[0189] After the completion of the lapping, the glass substrate wasimmersed and washed successively in the washing tanks of a neutraldetergent and water.

[0190] (4) First Polishing Step

[0191] The glass substrate was then subjected to a first polishing step.The first polishing step was carried out by a polishing machine in orderto remove the mars or distortions which had remained after the lappingstep.

[0192] Described specifically, the first polishing step was carried outunder the following polishing conditions by using a rigid polisher(“Cerium pad MHC15”, trade name; product of Speed Pham) as a polisher(polishing powder).

[0193] Polishing liquid: cerium oxide+water

[0194] Load: 300 g/cm² (L=238 kg)

[0195] Polishing time: 15 minutes

[0196] Removed amount: 30 μm

[0197] Rotational speed of lower platen: 40 rpm

[0198] Rotational speed of upper platen: 35 rpm

[0199] Rotational speed of inner gear: 14 rpm

[0200] Rotational speed of outer gear: 29 rpm

[0201] After completion of the first polishing step, the glass substratewas immersed successively in the washing tanks of a neutral detergent,pure water, pure water, IPA (isopropyl alcohol) and IPA (vapor drying),whereby washing was carried out.

[0202] (2) Second Polishing Step

[0203] A second polishing step was carried out using a polishing machineemployed in the first polishing step and a soft polisher (“Polirax”,trade name; product of Speed Pham) instead of the rigid polisher.Polishing was conducted under the similar conditions to the firstpolishing step except that the load, polishing time and removal amountwere changed to 100 g/cm², 5 minutes and 5 μm, respectively.

[0204] After the completion of the second polishing step, the glasssubstrate was immersed and washed successively in the washing tanks of aneutral detergent, a neutral detergent, pure water, pure water, IPA(isopropyl alcohol) and IPA (vapor drying), respectively. Incidentally,supersonic waves were applied to each of the washing tanks.

[0205] (6) Chemical Reinforcement Step

[0206] After the completion of the grinding, polishing and washingsteps, the glass substrate was subjected to chemical reinforcement.

[0207] The chemical reinforcement is carried out by pouring a chemicalreinforcement liquid into a chemical reinforcement tank and then dippinga retaining means having the glass substrate thereon in the chemicalreinforcement tank. At this time, the liquid in the chemicalreinforcement tank is being circulated by a pump and it is supplied tothe treatment tank after filtered and cleaned through a filter of astainless net which has a mesh of 1 micron or so and is disposed midwayin the circulation route. The stainless net serving as a filter makes itpossible to prevent the adhesion, onto the glass substrate, of ironpowders or stainless pieces, which have been generated from themanufacturing facilities or buildings, floated in the atmosphere andthen fallen in the chemical reinforcement tank, or glass chips generatedby the scratch of the glass substrate.

[0208] The glass substrate under such conditions was subjected tochemical reinforcement as described below: The glass substrate washedand heated in advance to 300° C. was immersed for about 3 hours in achemical reinforcement solution preliminarily heated to 400° C., saidsolution having been prepared by mixing potassium nitrate (60%) andsodium nitrate (40%). With a view to applying the chemical reinforcementall over the surface of the glass substrate, the chemical reinforcementwas effected with plural glass substrates being placed in a holder uponimmersion so as to retain them at the end faces.

[0209] When the glass substrate is immersed in a chemical reinforcementsolution, lithium ions and sodium ions on the surface layer of the glasssubstrate are substituted by sodium ions and potassium ions in thechemical reinforcement solution, respectively, whereby the glasssubstrate is reinforced.

[0210] The compression stress layer formed on the surface layer of theglass substrate had a thickness of about 100 to 200 μm.

[0211] After the completion of the chemical reinforcement, the glasssubstrate was quenched by dipping in a water tank of 20° C. and kept init for about 10 minutes.

[0212] After the completion of the quenching, the glass substrate wasimmersed in sulfuric acid heated to about 40° C. It was washed, whilesupersonic waves were applied.

[0213] The glass substrate so obtained had a surface roughness Ra of 0.5to 1 nm.

[0214] As a result of close examination of the glass surface, particleswhich would be a cause for thermal asperity were not observed. Inparticular, iron powders having a particle size of at least 3 to 5microns were not observed at all.

[0215] (7) Fabrication Step of Magnetic Disk

[0216] On both sides of the glass substrate for a magnetic disk obtainedthrough the above steps, an AlN texture layer, Cr primary layer, CrMoprimary layer, CoPtCrTa magnetic layer and C protective layer wereformed successively by an in-line system sputtering apparatus, whereby amagnetic disk was obtained.

[0217] As a result of the grinding test on the magnetic disk soobtained, neither hit nor crash was observed. It was also confirmed thatdefects attributable to particles which would be a cause for thermalasperity were not generated in a magnetic layer or the like layer.

[0218] Incidentally, the grinding test was conducted for comparison onboth the magnetic disk according to the present example obtained bychemical reinforcement with a clean chemical reinforcement solution anda magnetic disk according to comparative example obtained by chemicalreinforcement without a trapping means. As a result, it has been foundthat the number of inferior disks was by far smaller in the case of thepresent example.

[0219] After the grinding test, the magnetic disk according to thepresent example was subjected to a reproduction test by using amagnetoresistive type head. As a result, a malfunction in thereproduction due to thermal asperity was not recognized in any of the500 samples.

EXAMPLES 10-11

[0220] In Examples 10 and 11, in a similar manner to Example 9 exceptfor the use of a soda-lime glass (Example 10) and a soda aluminosilicateglass (Example 11) instead of the aluminosilicate glass, glasssubstrates for a magnetic disk and magnetic disks were prepared,respectively.

[0221] As a result, they were confirmed to show the same results as inExample 9.

EXAMPLE 12

[0222] On both sides of the glass substrate for a magnetic disk whichhad been obtained in Example 9, were formed by an in-line sputteringapparatus a primary layer composed of Al (film thickness: 50 angstrom),Cr (1000 angstrom) and CrMo (100 angstrom), a magnetic layer composed ofCoPtCr (120 angstrom), CrMo (50 angstrom) and CoPtCr (120 angstrom) anda protective layer composed of Cr (50 angstrom).

[0223] The resulting substrate was immersed in a solution of anorganosilicon compound (a mixed solution of water, IPA andtetraethoxysilane) having fine silica particles (particle size: 100angstrom) dispersed therein, followed by calcination, whereby aprotective layer composed of SiO₂ and having a texture function wasformed. The surface of the protective layer was subjected to diptreatment with a lubricant composed of perfluoropolyether to form alubricating layer thereon, whereby a magnetic disk for an MR head wasobtained.

[0224] As a result of the grinding test on the magnetic disk sofabricated, neither hit nor crash was observed. It was also confirmedthat no defect was caused in the film such as magnetic layer. As aresult of the reproduction test by using a magnetoresistive type head, amalfunction in the reproduction attributable to thermal asperity was notobserved.

EXAMPLE 13

[0225] In a similar manner to Example 12 except for the use of anprimary layer formed of Al, Cr and Cr and a magnetic layer formed ofCoNiCrTa instead, a magnetic disk for a thin-film head was obtained.

[0226] The magnetic disk so obtained was confirmed to show the sameresults as in Example 12.

[0227] In the above example, a filter was employed as a means fortrapping the particles in the chemical reinforcement liquid.Alternatively, it is possible to dispose a magnet or the like in thechemical reinforcement solution; or to dispose it in contact with aninner wall of the chemical reinforcement tank or bury it in the innerwall in order to trap only iron powders.

EXAMPLE 14

[0228] (1) Rough Grinding Step

[0229] A glass substrate formed of an aluminosilicate glass cut from asheet glass, which had been formed by the down-drawing method, into adisk having a diameter of 66 mmØ and a thickness of 3 mm by a grindingstone was ground by a comparatively rough diamond grinder to a diskhaving a diameter of 66 mmØ and a thickness of 1.5 mm.

[0230] Instead of the down-drawing method, it is possible to employ thedirect pressing method to press a fused glass into a glass disk by usingan upper mold, lower mold and cylindrical mold. Alternatively, thefloating method may be used.

[0231] Incidentally, as the aluminosilicate glass, used was a glass forchemical reinforcement containing as principal components 57 to 74% ofSiO₂, 0 to 2.8% of ZnO₂, 3 to 15% of Al₂O₃, 7 to 16% of Li₂O and 4 to 14wt. % of Na₂O, each in terms of mole %, (for example, a glass forchemical reinforcement containing as principal components 67.0% of SiO₂,1.0% of ZnO₂, 9.0% of Al₂O₃, 12.0% of Li₂O and 10.0% of Na₂O, each interms of mole %).

[0232] Both sides of the glass substrate were ground by a diamondgrinder having a particle size smaller than the above grinder,respectively. The load at that time was about 100 kg, by which bothsides of the glass substrate were each formed to have a surfaceroughness of about 10 μm in terms of Rmax (as measured in accordancewith JIS B 0601).

[0233] By using a cylindrical grinding stone, a hole was made at thecenter of the glass substrate and the outer peripheral end face wasground so that the glass substrate had a diameter of 65 mmØ. The outerperipheral end face and inner peripheral face were subjected to apredetermined chamfering treatment. The end face of the glass substrateat that time had a surface roughness of about 4 μm in terms of Rmax.

[0234] (2) Mirror-Finish Processing Step of End Face

[0235] While the glass surface was allowed to rotate, its end face wassubjected to brush polishing to have a surface roughness of about 1 μmin terms of Rmax and about 0.3 μm in terms of Ra.

[0236] After the completion of the mirror-finish processing of the endface, the surface of the glass substrate was washed with water.

[0237] (3) Lapping Step

[0238] The glass substrate was then subjected to a lapping step. Thelapping step aims at the improvement in the size accuracy and shapeaccuracy. The lapping was carried out by a lapping apparatus twice bychanging the grain size of an abrasive from #400 and #1000.

[0239] Described specifically, with an alumina abrasive having aparticle size of #400 at a load L of about 100 kg, both sides of theglass substrate placed in a carrier were lapped to have an arealaccuracy of 0 to 1 μm and a surface roughness (Rmax) of 6 μm or so byallowing the inner gear and outer gear to rotate.

[0240] The lapping was then effected by changing the grain size of analumina abrasive to #1000, whereby the surface roughness was improved toabout 2 μm (Rmax).

[0241] After the completion of the lapping, the glass substrate wasimmersed and washed successively in the washing tanks of a neutraldetergent and water.

[0242] (4) First Polishing Step

[0243] The glass substrate was then subjected to a first polishing step.The first polishing step was carried out by a polishing machine in orderto remove the mars or distortions which had remained after the lappingstep.

[0244] Described specifically, the first polishing step was carried outunder the following polishing conditions by using a rigid polisher(“Cerium pad MHC15”, trade name; product of Speed Pham) as a polisher(polishing powder).

[0245] Polishing liquid: cerium oxide+water

[0246] Load: 300 g/cm² (L=238 kg)

[0247] Polishing time: 15 minutes

[0248] Removed amount: 30 μm

[0249] Rotational speed of lower platen: 40 rpm

[0250] Rotational speed of upper platen: 35 rpm

[0251] Rotational speed of inner gear: 14 rpm

[0252] Rotational speed of outer gear: 29 rpm

[0253] After completion of the first polishing step, the glass substratewas immersed successively in the washing tanks of a neutral detergent,pure water, pure water, IPA (isopropyl alcohol) and IPA (vapor drying),whereby washing was carried out.

[0254] (5) Second Polishing Step

[0255] A second polishing step was carried out using a polishing machineemployed in the first polishing step and a soft polisher (“Polirax”,trade name; product of Speed Pham) instead of the rigid polisher.Polishing was conducted under the similar conditions to the firstpolishing step except that the load, polishing time and removal amountwere changed to 100 g/cm², 5 minutes and 5 μm, respectively.

[0256] After the second polishing step, the glass substrate was washed.Steps from the washing step to the step of packing in a case werecarried out under an environment of the clean air supplied from a cleanboth. The glass substrate was immersed and washed successively in thewashing tanks of a neutral detergent, a neutral detergent, pure water,pure water, IPA (isopropyl alcohol) and IPA (vapor drying).Incidentally, supersonic waves were applied to each of the washingtanks.

[0257] (6) Washing with Hydrochloric Acid

[0258] The glass substrate was then washed with hydrochloric acid todissolve and remove fine iron contaminants adhered to the surface, innerperipheral face and outer peripheral surface of the glass substrate.Washing of a plural numbers of glass substrates supported was conductedby dipping in hydrochloric acid (hydrochloric acid:water=10 Kg:43 liter)contained in the washing tank filtered and circulated by a 1.2 micronmesh type filter (about 10 minutes).

[0259] In this manner, iron contaminants are removed prior to thechemical reinforcement, whereby under-layer defects can be prevented. Itis particularly important to carry out washing with hydrochloric acidprior to the chemical reinforcement. Because when the chemicalreinforcement is carried out with iron contaminants being adhered on tothe glass substrate, the surface portion of the glass substrate underthe iron contaminants remains unreinforced and this unreinforced portionwill become under-layer defects. The above-described washing withhydrochloric acid can prevent the generation of such under-layerdefects.

[0260] (7) Chemical Reinforcement Step

[0261] After the washing step, the glass substrate was subjected tochemical reinforcement. The chemical reinforcement is carried out bypouring a chemical reinforcement liquid into a chemical reinforcementtank and then immersing the glass substrate supported by a retainingmember in the chemical reinforcement liquid. Incidentally, the retainingmember for the glass substrate is formed of three supports which form aplurality of V-grooves at equal intervals in the arranging direction ofthe glass substrate, said three supports being connected at each endface thereof with a connecting member. Each glass substrate is supportedat three points by the V-grooves within the same plane of three supportsand plural glass substrates are arranged in the extending direction ofthese supports.

[0262] The supports and connecting members of the retaining member ofthis embodiment are each formed of SUS316 which is an austeniticstainless alloy having excellent corrosion resistance in a hightemperature region required upon chemical reinforcement. The chemicalreinforcement tank is, on the other hand, formed of SUS304 which is alsoan austenitic stainless alloy. The chemical reinforcement tank andretaining means may be formed of the same material or differentmaterials. As another stainless alloy, SUS316L is suited. The abovestainless alloys (SUS316, SUS304, SUS316L) are defined by JapaneseIndustrial Standards (JIS G 4303-JIS G 4305). The chemical reinforcementliquid used in the present example is maintained clean because it iscirculated through a filter.

[0263] Described specifically, the chemical reinforcement was carriedout as follows. The glass substrate which had been washed was heated inadvance to 300° C. and immersed for about 3 hours in a chemicalreinforcement solution preliminarily heated to 400° C., said solutionhaving been prepared by mixing potassium nitrate (60%) and sodiumnitrate (40%). With a view to applying the chemical reinforcement allover the surface of the glass substrate, the chemical reinforcement waseffected with plural glass substrates being placed in a holder uponimmersion so as to retain them at the end faces.

[0264] When the glass substrate is immersed in the chemicalreinforcement solution, lithium ions and sodium ions on the surfacelayer of the glass substrate are substituted by sodium ions andpotassium ions in the chemical reinforcement solution, respectively,whereby the glass substrate is reinforced.

[0265] Since the compression stress layer formed on the surface layer ofthe glass substrate had a thickness of about 100 to 200 μm. The chemicalreinforcement tank and the retaining member for the glass substrate,which were brought into contact with the hot chemical reinforcementliquid upon chemical reinforcement, were each formed of an austeniticstainless alloy having excellent chemical durability; and the chemicalreinforcement liquid was circulated through a filter, adhesion of metalpieces such as iron powder or chromium and metal oxides to the glasssubstrate during the chemical reinforcement could be prevented.

[0266] After the completion of the chemical reinforcement, the glasssubstrate was quenched by dipping in a water tank of 20° C. and kept init for about 10 minutes, whereby inferior products impaired with finecracks can be removed.

[0267] (8) Washing and Packing Steps

[0268] After quenching, the glass substrate was immersed in sulfuricacid heated to about 140° C. and washed under the application ofultrasonic waves. By the above washing with sulfuric acid, the elutionof alkali ions from the glass substrate can be prevented and besides,the precipitation salt on the glass substrate by the chemicalreinforcement liquid can be removed. The glass substrate was thensubjected to final washing, vapor drying and packing in a case.

[0269] After the above-described steps, the principal surface of theglass substrate had a surface roughness Ra of 0.5 to 1 nm.

[0270] As a result of close examination of the glass surface, particleswhich would be a cause for thermal asperity were not observed. Inparticular, fine iron powders having a particle size of at least 0.1 to5 μm were not observed at all. In the present example, not only washingwith hydrochloric acid but also cleanliness control of the environmentand chemical reinforcement liquid was conducted to remove ironcontaminants so that the iron contaminants could be removed almostcompletely.

[0271] Incidentally, washing with hydrochloric acid was conducted priorto the chemical reinforcement in the present example but it is alsopossible to carry out the washing subsequent to the chemicalreinforcement or washing with sulfuric acid. Alternatively, washing withhydrochloric acid may be effected in all the steps prior to the chemicalreinforcement, subsequent to the chemical reinforcement or subsequent towashing with sulfuric acid. It may be effected in plural steps selectedin combination.

[0272] (9) Fabrication Step of Magnetic Disk

[0273] On both sides of the glass substrate for a magnetic disk obtainedthrough the above steps, an AlN texture layer, Cr primary layer, CrMoprimary layer, CoPtCrTa magnetic layer and C protective layer wereformed successively by an in-line system sputtering apparatus, whereby amagnetic disk was obtained.

[0274] As a result of the grinding test on the magnetic disk soobtained, neither hit nor crash was observed. It was also confirmed thatno defects were caused in the film such as magnetic layer by theparticles which would otherwise cause thermal asperity.

[0275] Incidentally, the substrate, as obtained in the present example,from which iron contaminants had been removed by washing withhydrochloric acid and the substrate obtained in comparative examplewithout hydrochloric acid treatment were compared. As a result, a numberof fine iron powders having a particle size of 10 to 80 microns wereobserved on the surface of the glass substrate obtained in thecomparative example. The superiority of the present example can beunderstood from the comparison in the results between the presentexample and comparative example.

[0276] After the grinding test, the magnetic disk was subjected to areproduction test by using a magnetoresistive type head. As a result, amalfunction in the reproduction attributable to thermal asperity was notobserved from each of the plural samples (500 disks).

EXAMPLES 15-16

[0277] In Examples 15 and 16, in a similar manner to Example 14 exceptfor the use of soda-lime glass (Example 15) and soda aluminosilicateglass (Example 16) instead of the aluminosilicate glass, glasssubstrates for a magnetic disk and magnetic disks were obtained,respectively.

[0278] As a result, a chemically reinforced glass, similar to thatobtained in Example 14, having a surface free from metallic pieces suchas iron powders was obtained.

[0279] The kind of the glass was changed to acrystallized glass and theglass was washed with hydrochloric acid after polishing without chemicalreinforcement and washing steps. In that case, similar effects werebrought about for the removal of iron contaminants as in the aboveexample.

EXAMPLE 17

[0280] On both sides of the glass substrate for a magnetic disk, whichhad been obtained in Example 14, were formed by an in-line sputteringapparatus a primary layer composed of Al (film thickness: 50 angstrom),Cr (1000 angstrom) and CrMo (100 angstrom); a magnetic layer composed ofCoPtCr (120 angstrom), CrMo (50 angstrom) and CoPtCr (120 angstrom) anda protective layer composed of Cr (50 angstrom).

[0281] The resulting substrate was immersed in a solution of anorganosilicon compound (a mixed solution of water, IPA andtetraethoxysilane) having fine silica particles (particle size: 100angstrom) dispersed therein, followed by calcination, whereby aprotective layer composed of SiO₂ and having a texture function wasformed. The surface of the protective layer was subjected to diptreatment with a lubricant composed of perfluoropolyether to form alubricating layer thereon, whereby a magnetic disk for an MR head wasobtained.

[0282] As a result of the grinding test on the magnetic disk sofabricated, neither hit nor crash was observed. It was also confirmedthat no defect appeared in the film such as magnetic layer. As a resultof the reproduction test by using a magnetoresistive type head, amalfunction in the reproduction attributable to thermal asperity was notobserved.

EXAMPLE 18

[0283] In a similar manner to Example 17 except that the primary layerwas formed of Al, Cr and Cr and the magnetic layer was formed ofCoNiCrTa, a magnetic disk for a thin-film head was fabricated.

[0284] The magnetic disk so obtained was confirmed to show the sameresults as that obtained in Example 17.

[0285] The present invention was described above by the preferredexamples but it should however borne in mind that the present inventionis not limited to or by the above examples. For example, the kind of theglass substrate or magnetic layer is not limited to those employed inthe above examples.

[0286] As described above, a glass substrate for an informationrecording medium free from the adhesion of particles such as fine ironpowders can be obtained according to the present invention. By formingan information recording layer and the like on the glass substrate soobtained, a high-quality information recording medium free fromunder-layer defects can be obtained.

What is claimed is:
 1. A process for producing a glass substrate for aninformation recording medium, which comprises: (a) a step of polishing aprincipal surface of a glass substrate; and (b) a step of carrying outchemical reinforcement of said glass substrate by bringing said glasssubstrate into contact with a chemical reinforcement liquid tosubstitute a portion of ions contained in said glass substrate by ionshaving a diameter larger than the former ions in said chemicalreinforcement liquid, wherein at least one of the steps subsequent to afinal polishing step, that is, a washing step, said chemicalreinforcement step, a step of washing off said chemical reinforcementliquid, a drying step, an inspection step, and a step of packing saidglass substrate or filling the same in a container is effected in anatmosphere where a clean filtered air is circulated so as to prevent theadhesion of particles to said glass substrate.
 2. A process forproducing a glass substrate for an information recording medium, whichcomprises: (a) a step of polishing a principal surface of a glasssubstrate; and (b) a step of carrying out chemical reinforcement of saidglass substrate by bringing said glass substrate into contact with achemical reinforcement liquid to substitute a portion of ions containedin said glass substrate by ions having a diameter larger than the formerions in said chemical reinforcement liquid, wherein at least one of thesteps subsequent to a final polishing step, that is, a washing step,said chemical reinforcement step, a step of washing off said chemicalreinforcement liquid, a drying step, an inspection step, and a step ofpacking said glass substrate or filling the same in a container iseffected under an environment whose air cleanliness permits theexistence of particles having a size of 0.3 to 100 μm in an amount notmore than 1000 particles per cubic feet·meter of the air.
 3. A processaccording to claim 1 , wherein the clean filtered air has cleanlinesspermitting the existence of particles having a size of 0.3 to 100 μm inan amount not more than 1000 particles per cubic feet·meter of the air.4. A process according to claim 1 or 2 , wherein the air has cleanlinesspermitting the existence of particles in an amount not more than 100particles per cubic feet·meter of the air.
 5. A process according toclaim 1 or 2 , wherein the air has cleanliness permitting the existenceof particles in an amount not more than 50 particles per cubicfeet·meter of the air.
 6. A process according to claim 1 or 2 , whereinthe washing and chemical reinforcement of the glass substrate after thefinal polishing step are effected under an environment whose aircleanliness permits the existence of particles in a prescribed amount inthe air.
 7. A process for producing a glass substrate for an informationrecording medium, which comprises a step of carrying out chemicalreinforcement of a glass substrate by bringing said glass substrate,which has been retained by a retaining means, into contact with achemical reinforcement liquid to substitute a portion of ions containedin said glass substrate by ions having a diameter larger than the formerions in said chemical reinforcement liquid, wherein at least a wallsurface of a chemical reinforcement tank containing said chemicalreinforcement liquid therein and/or said retaining means which is incontact with said chemical reinforcement liquid is formed of a stainlessalloy having corrosion resistance in a high-temperature region as highas the heating temperature of said chemical reinforcement liquid.
 8. Aprocess according to claim 7 , wherein said stainless alloy is amartensitic or austenitic stainless alloy.
 9. A process for producing aglass substrate for an information recording medium, which comprises astep of carrying out chemical reinforcement of a glass substrate bybringing said glass substrate into contact with a chemical reinforcementliquid to substitute a portion of ions contained in said glass substrateby ions having a diameter larger than the former ions in said chemicalreinforcement liquid, wherein a means for trapping fine particlesexisting in said chemical reinforcement liquid is provided.
 10. Aprocess according to claim 9 , wherein said means for trapping fineparticles is a filter for filtering said chemical reinforcement liquidcirculated.
 11. A process according to claim 9 , wherein said fineparticles are fine iron powders and said trapping means is a magnetdisposed to be in contact with said chemical reinforcement liquid.
 12. Aprocess for producing a glass substrate for an information recordingmedium, said glass substrate having a principal surface subjected toprecision polishing, which comprises a step of washing said glasssubstrate with hydrochloric acid.
 13. A process according to claim 12 ,which comprises a step of carrying out chemical reinforcement of a glasssubstrate by substituting a portion of ions contained in said glasssubstrate by ions having a diameter larger than the former ions, whichfurther comprises a step of washing said glass substrate withhydrochloric acid as a pre-treatment or post-treatment step of saidchemical reinforcement step.
 14. A process according to any one ofclaims 1, 2, 7, 9 and 12, wherein said glass substrate for aninformation recording medium is a glass substrate for a magnetic disk.15. A glass substrate according to claim 14 , wherein said glasssubstrate for a magnetic disk is a glass substrate for a magnetic diskfor a magnetoresistive type head.
 16. A process for producing aninformation recording medium, which comprises forming at least arecording layer on the glass substrate obtained by the process asclaimed in any one of claims 1, 2, 7, 9 and
 12. 17. A process accordingto claim 16 , wherein said recording layer is a magnetic layer.